The Basics

No current flows in or out of the inputs.  (Well actually a small bias current does, but its small)

Output goes positive if the + input is more positive than the – input.

Output goes negative if the + input is more negative than the – input.

Output will do whatever is necessary to make the inputs equal.

OpAmp Selection / Design Specs

Rail To Rail

Rail to rail may mean the op amp can operate totally rail to rail, OR just the inputs can accept rail to rail but the output can’t OR the output can but the inputs can’t – watch out and always check the spec!

Input common-mode voltage range (input common-mode range) (Vicmr)

VICMR defines a range of common-mode input voltages that result in proper operation of the op amp and describes how close the inputs can get to either supply rail.

Another way to think of VICMR is that it describes a range which is defined as VICMR_MAX – VICMR_MIN 

If Vicmr is exceeded the normal linear operation of the op amp is no longer guaranteed.  You may get clipping, the output jumping to a supply rail suddenly or other undefined behavior.  This is a classic mistake often made in single supply op amp applications.

Vicmr is quite different from op amp to op amp and may fall within or beyond the supply rails. Never assume that an op amp can receive a particular input signal range without verifying it in the datasheet specifications.

Good resources:

planetanalog.com/document.asp?doc_id=528175

Input Bias Current

One of the golden rules of op amps says that no current flows into either input terminal. However, in reality, a small current flows into both inputs to bias the input transistors. Unfortunately, this bias current gets converted into a voltage by the circuit’s local resistors and amplified right along with the signal. The result is an output error in your circuit.

This can be an issue for:

  • High impedance input sources, fore these you often want as low as possible input bias current.
  • Peak hold circuits, where you don’t want to discharge the holding capacitor any more than absolutely necessary.
  • Sensitive inputs with weak pull resistors to say a Vmid voltage. The OpAmp input acts as an additional pull resistor in effect causing your output voltage to not be centred around Vmid unless you reduce the pull resistor value(s), which of course reduces the sensitivity of your input.

For example, a LMV358 with an input 470K resistor to 2.5V outputed 2.87V in a circuit we worked on that had an op amp amplification of x17, so the bias current was adding 21.8mV to the input voltage. Reducing the pull resistor to 100K removed virtually all of the error, but also reduced the input sensitivity.

What can you do about it?

  • Select an OpAmp with a lower input bias current specification. Bias current specification can range from μA down to pA
  • A clever choice of resistor values can help you cancel most of the output error, but there will always be an element of tolerance at play as the input bias current can vary slightly.

Design Considerations

Opamps are very stable with temperature.

JFET (i.e. TL084) inputs can be bad for oscillations / noise as they are high impedance.  Usually need to use with resistor and capacitor on input.

Dual Rail Opamps From A Single Rail Supply

A typical opamp only has + and – power pins so you can use a dual rail op amp connected to a +V and 0V supply.  The dual rail op amp design simply assumes GND it is half-way between its two power supplies.   The main issue is typically circuitry around the opamp, but if you are constructing a fake GND that is half way between the supply rails, say a Vmid scenario where you use a potential divider, then using a dual rail opamp can be fine.

Voltage Follower

A voltage follower has the output connected to ‘-‘ and the input voltage connected to ‘+’.

Unused Inputs

Wire as a follower (output to ‘-‘) but with ‘+’ input tied to a mid votlage (0V for dual supply, between 2 resistors for single supply).  If you have a single supply and don’t want to fit resistors then you can connect the ‘+’ input to 0V, but the op amp will consume more current and analog purists will berate you.  Alternatively if you have a voltage rail sitting somewhere between the op amps +V & – V then you can use that.

Bad Approaches

  • Using a resistor to 0V is not really a better solution.
  • Tying both inputs together can be bad due to randomness in offset votlages in an op amp
  • Tying inputs high and low can over stress some inputs and causes more current consumption.
Good resources

http://www.maxim-ic.com/appnotes.cfm/an_pk/1957

Good Op Amp Resources

EEVBlog OpAmps Explained

OpAmps We Often Use

Single, Dual & Quad OpAmp​s, Single Rail

LMV341​

2.7 – 5.5V, rail to rail, low power, high temperature

AD8541

General purpose, rail to rail, 2.7V – 5.5V, low running current,
1MHz, instrumentation, sensors, audio

AD8531

General purpose, 250mA output, rail to rail, 2.7V – 5.5V,
Low running current, 1MHz,

Single, Single Rail

LMV321

General purpose, rail to rail, 2.7V – 6V, low running current, low cost

LMV721

Low noise, 2.2 – 5V, rail to rail, low power, 10MHz

 

Dual OpAmp, Single Rail

LM358

General purpose, low power, swings GND to (VCC – 1.5V), use with logic systems, 3 to 30V

LMV358

General purpose, low voltage, rail to rail output

LMV722

Fast (10MHz), low voltage, rail to rail

LMV722

Low noise, 2.2 – 5V, rail to rail, low power, 10MHz

Quad OpAmp, Single Rail

LM324

General purpose, swings GND to (VCC – 1.5V).
Quite high output drive

LMV324

General purpose, rail to rail, 2.7V – 6V, low running current, low cost

Single, Dual & Quad OpAmp​s, Dual Rail

TL064

JFET Lower power version of TL084

TL084

JFET input OpAmp

Dual OpAmp, Dual Rail

NE5532

High power output

Precision Op Amps

AD8628

Rail To Rail, zero drift

High Output Current

LM8261

Rail To Rail

Low Bias Current

TLV6001U

Single, 1pA bias current, rail to rail, 1.8 to 5.5VCC

LTC6244

Dual, 1pA bias current

 

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